Conventionally, in a vehicle having an engine as a source of driving force, an automated manual transmission (hereinafter referred to as an AMT) has been known. According to the AMT, various actuators are added to an existing manual transmission (gear shifting transmission) for automatically operating a series of gear shifting operations such as clutch engagement/disengagement, gear shifting, gear selecting, in response to an intension or request of an operator of the vehicle or depending on a vehicle running state.
In such AMT, when the clutch is disengaged, first, master cylinder is operated by a clutch actuator, of which drive source is a motor and by the operation of the master cylinder, an idle port is closed to operate a friction clutch slave cylinder (hydraulic direct cylinder). Then the diaphragm spring is deformed by the operation of the slave cylinder to reduce the pressurized load of the clutch disc which is applied on the flywheel. In other words, under the clutch being in disengagement state, the idle port of the master cylinder is closed to interrupt the fluid communication between the master cylinder and the slave cylinder by closing a communication passage provided therebetween.